1. Field of the Invention
The present invention relates to a blood-pressure monitor apparatus which monitors the blood pressure of a living subject.
2. Related Art Statement
The blood pressure of a living subject such as a patient may continuously be monitored by using an automatic blood-pressure (BP) measuring device including an inflatable cuff adapted to be wound around a body portion (e.g., upper arm) of the subject. In this case, the BP measuring device periodically carries out BP measuring operations at a regular interval of time. However, if this interval is shortened to improve the reliability of the BP monitoring, then the frequency of pressing of the cuff against the subject's body portion increases, thereby causing the subject to feel even serious discomfort.
In contrast, a different BP monitoring method is disclosed in Japanese Patent Applications laid open for inspection under Publication Nos. 61-103432 and 60-241422. In this method, an inflatable cuff is wound around a body portion of a living subject and is inflated to apply an appropriate pressing pressure to the body portion, and a pulse-wave sensor continuously detects heartbeat-synchronous pulses produced as pressure oscillations in the cuff. The disclosed BP monitor device continuously estimates the BP values of the subject based on the respective amplitudes of the continuous pulses.
However, in the above-described BP monitor device, the pressing pressure of the cuff cannot be decreased to sufficiently low levels to be able to reduce the physical and/or psychological load to the subject, because the changing of the pulse amplitudes at such low levels does not accurately correspond to the changing of the subject's blood pressure. A curve indicated at solid line in FIG. 35 represents the envelope of the pulse amplitudes which are obtained from the cuff as the cuff pressure Pc is changed. If the blood pressure of the subject decreases from a normal level represented by the solid-line curve, the solid-line curve is changed into a curve indicated at one-dot chain line in the same graph. In the case where the pulse amplitudes are detected at a low cuff pressure, P.sub.K, the amount of changing of the pulse amplitudes is significantly smaller than that of the subject's blood pressure, thereby lowering the accuracy of monitoring of the prior BP monitor device.